It is common for hydraulic pipes and other systems to be routed through the inside of an aircraft wing and to be attached to its inner surface. Similarly, components such as heat exchangers often need to be mounted to the inside surface of the wing. As the wing is effectively a fuel tank, the pipes, systems or other components may be immersed in fuel or, at the very least, exposed to fuel vapour.
It is conventional for pipes, systems and components to be mounted on brackets that are mounted on the inside of the skin of an aircraft, in particular on the inner surface of the skin covering the wing using nuts and bolts that pass through holes in the skin and the component or bracket. Ideally, these bolts are a clearance fit through the holes in the skin and the holes in the component or bracket to ensure that the components or pipes can be easily released by undoing the bolts during maintenance or replacement.
When lightening strikes an aircraft wing, the metal outer skin acts as a Faraday cage and so protects the components mounted within the wing from damage and, more importantly, dissipates the energy from the lightening strike away from the fuel. However, it has recently become more common to manufacture the skin of an aircraft wing out of a carbon composite material rather than metal. This reduces its ability to dissipate the energy generated as a result of a lightening strike and has resulted in a problem known as ‘out-gassing’ in which lightening can track down the clearance gap between a fastener and the skin in which it is received and create hot plasma in a region surrounding the nut. This is a source of ignition that needs to be avoided. Metallic pipes & components are distanced from the inner surface of the skin to reduce the likelihood of lightning energy jumping the gap between the pipe or component & the skin, causing a spark.
It is known to provide a cap or cover that fits over the nut and contain any plasma generated in the region surrounding the nut. However, this additional component may inadvertently come off the nut. Not only would this result in there being no protection against out gassing, but a loose cap in the fuel tank could block fuel flow pipes and filters. Because of its inaccessiblity within the fuel tank, it is also difficult to detect whether any caps have come loose.
One way of alleviating this problem is to make the bolts an interference fit within the aircraft wing so there is no longer any gap for lightening to track down. However, it then becomes difficult to remove the bolts at a later date in order to release the components being held by them.